A liquid crystal display device essentially includes a polarizing plate, which is typically constituted of a polarizer and a protective film bonded to the polarizer. Typically, a triacetylcellulose film is used as the protective film for the polarizer. However, a thin triacetylcellulose film will provide insufficient mechanical strength and has high water permeability, causing deterioration of the polarizer. In addition, since the triacetylcellulose film is expensive, there is a need for inexpensive alternative materials.
A polyester film is more inexpensive and has higher mechanical strength than the triacetylcellulose film. However, when used as a protective film of the polarizer, the polyester film has a problem of rainbow mura due to birefringence thereof. Accordingly, the polyester film is stretched at high elongation to increase in-plane retardation in order to prevent the rainbow mura from being visible.
In recent years, in an attempt to improve image quality and light use efficiency, a light source having a high color gamut of about 95% or more by the NTSC (National Television System Committee) is used instead of a white light source having a color gamut of about 60% to about 70% (for example, YAG (yttrium aluminum garnet) phosphors included). However, when applied to a liquid crystal display device including such a high color gamut light source, the birefringent polyester film having high in-plane retardation allows the rainbow mura to be viewed and a typical polyester film having low in-plane retardation allows the rainbow mura to be severely viewed due to birefringence of the polyester film. Therefore, there is a need for a method for preventing the rainbow mura from being visible even in application of a high color gamut light source.
The background technique of the present invention is disclosed in Japanese Unexamined Patent Publication No. 2014-044387.